Sternomastoid muscle fibers of a living mouse. YOYO-1 and fluorescein di-beta-D-galactopyranoside (FDG).

Sternomastoid muscle fibers of a living mouse that have been transfected with YOYO-1 dye–stained DNA (red) containing the lacZ reporter gene and then stained with the beta-galactosidase substrate fluorescein di-beta-D-galactopyranoside (FDG, F1179). DNA stained with YOYO-1 (Y3601) prior to implantation could still be localized five days after application. Fluorescence signals were visualized in situ by epifluorescence microscopy with a low–light level SIT camera and a computer imaging system. Image contributed by Peter van Mier, Washington University School of Medicine.